In the related art, a printer forms an image using a coloring layer (leuco layer) formed on a sheet. The image can be formed on the sheet by heating the coloring layer, and the image can be erased by additionally heating the coloring layer. Since the image can be erased from the sheet, the sheet can be recycled many times for printing. One type of such a printer uses sheets that have the coloring layer formed on the entire region thereof. The sheets are usually fixed size and stored in a sheet storage unit of the printer, and the printer takes the sheets one by one for printing. However, the images are not necessarily formed in the entire region of the sheets. When the images are formed only on a specific portion of the sheets, the coloring layer on the other region of the sheets may be wasted.